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I am trying to understand, between acetamide, propionamide and butyramide, which compound has greater/weaker electronegative oxygen in their respective carbonyl groups.

It is known that alkyl groups have an electron-donating inductive effect, does this mean that acetamide will have the most electronegative carbonyl oxygen atom?

To be more precise, I am trying to develop an ad-hoc model to run in classical molecular dynamics (MD) simulations. For this purpose, I want to obtain the partial charges on the oxygen atom in the carbonyl group.

In the CHARMM forcefield 1, this value is taken to be -0.55e for acetamide. However, I am trying to generalize this forcefield for propanamide and butyramide by considering how the charge will redistribute due to the addition of alkyl groups in the molecule.

1 J Comput Chem. 2010 Mar; 31(4): 671–690 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2888302/)

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    $\begingroup$ One could talk about partial charge on oxygens. Electronegativity? Not really. $\endgroup$
    – Mithoron
    Commented Jul 1 at 14:20
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    $\begingroup$ The big contributor is the amide resonance. $\endgroup$
    – user55119
    Commented Jul 1 at 18:09
  • $\begingroup$ @Mithoron Thank you for pointing this out. I have clarified it in the revised question. $\endgroup$
    – user35952
    Commented Jul 2 at 10:35
  • $\begingroup$ @user55119 Would you mind elaborating more on that? In particular, how it could alter the partial charge on oxygen? $\endgroup$
    – user35952
    Commented Jul 2 at 10:36
  • $\begingroup$ I was about to look up the carbonyl frequencies of the 3 amides you mentioned. Unfortunately, the SDBS database (sdbs.db.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi) is down for maintenance.. If the length of the alkyl chain (Me, Et, Pr) had an impact, you should see it in carbonyl frequency but I doubt it. Resonance>>>Inductive Effect. $\endgroup$
    – user55119
    Commented Jul 2 at 15:11

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